In heavy camouflage gear, Johnny Cochran squats down and shuffles noiselessly along the ground. His target is a large man who, like Cochran, is in military fatigues. Seconds later, Cochran leaps up and stabs the man once, hard, in the neck. The movement is swift, and would almost certainly be lethal, were it not for the fact that the ‘weapon’ Cochran is wielding is a pen.

The scenario I have just witnessed may be simulated, but its protagonists are deadly serious. This is a ‘close combat training’ session given by ‘Fireteam Diamondback’ – an armed militia group, or civilian ‘army’, based in west Texas, in the United States. Cochran, a chain-smoking 39 year-old with a handlebar moustache and goatee whose T-shirt reads: ‘Disgruntled Combat Vet – Right Wing Extremist’, is their leader. Biro-wielding or not, he’s not someone you would wish to encounter in combat.

‘Straight into the base of his skull,’ he says, after pretending to plunge the pen into the neck of Steven Page, a member of another militia group who has joined the training. ‘That’s the nerve centre. Then you push forward. If you’re dealing with someone short, that works like a charm, but if you’re dealing with someone tall, grab his face, insert the knife and when you shove that knife forwards, pull him towards you.’

Cochran smiles. ‘You’re going to make a hell of a mess, but human flesh tears easily. Bone is a pain in the a--.’ He knows what he’s talking about, having served four years as a combat medic with the US Marines during Operation Desert Storm. His ‘handle’, or nickname, in the militia is ‘Doc’. And yet, as he freely admits, the hypothetical enemy – the target he’s teaching the people gathered here today to kill – is a US soldier.

Why? Cochran says he is simply exercising his constitutional right to assemble an armed civilian force that is prepared to fight any enemy, be they domestic or foreign. There are 27 men in Cochran’s squad including, apparently, both former and serving soldiers, policemen and members of the sheriff’s department.

This didn’t surprise me. I’d already read about Richard Mack, a former sheriff of Graham County, Arizona, who now travels the country ‘crusading for freedom and individual rights’ and insists ‘the greatest threat we face today is not terrorists; it is our own federal government’.

The militias, which are dotted throughout the US and, according to recent figures, are growing rapidly in numbers, claim they are bulwarks against tyranny. The US Department of Homeland Security takes a dimmer view, warning of a ‘rise in Right-wing anti-government extremist activity’ as far back as April 2009 and a ‘phenomenon of violent radicalisation’.

Indeed, according to the Southern Poverty Law Centre (SPLC), which tracks extremist groups, the US has seen a dramatic spike in attempted domestic terrorism ever since Barack Obama started his campaign for office, including: two skinhead plots to assassinate him; a plan to set off a dirty bomb packed with radioactive materials during the inauguration; and a lone assassin, Keith Luke, who began murdering black people in Massachusetts.

Of course, militia activity is hardly new to the US. The very first article of the Constitution granted Congress the power to call on ‘the militia to execute the laws of the union, suppress insurrections and repel invasions’ and the subsequent Militia Act of 1792 defined the militia as every able-bodied male citizen over 18 and under 45.

It’s an act that has been embraced by a fair number of American citizens ever since, whether loners or disparate groups of armed, disgruntled civilians. In 1992, Randy Weaver, a former US Army Green Beret, moved himself and his family to an isolated cabin in Ruby Ridge, Idaho, to escape what they saw as a corrupt world. Rather than a peaceful nature-lover, officials claimed Weaver was a member of a race-hate group and he was charged with weapons violations. When he failed to appear in court, they stormed the cabin, resulting in the fatal shooting of Weaver’s wife, Vicki, and 14-year-old son, Sammy.

The deadly ambush only added to militias’ grievances against what they saw as an unlawful and despotic federal government. But it was the siege at Waco, Texas, a year later, that really ignited the movement. Following the deaths of 80 people in the fire, Waco became a rallying point for conspiracy theorists, members of the patriot movement and a rabid end-of-days philosophy.

Timothy McVeigh was one of those unhinged people who visited Mount Carmel during the weeks following the battle between cult members and officers of the Bureau of Alcohol, Tobacco and Firearms. Two years later, he would use plastic explosives to blow up the Federal Murrah building in Oklahoma City, killing 168 people.

It was 1995 before Theodore Kaczynski, the ‘Unabomber’, was arrested, after a domestic mail bombing campaign spanning almost 20 years. Having honed his survival skills from his isolated cabin outside Lincoln, Montana, the by-then 68 year-old had succeeded in killing three people and injuring 23 others with bombs that often included bits of treebark and wood, a symbolic protest against what he saw as the destruction of the wilderness around his home.

With this sort of history, it’s perhaps unsurprising that so many militias were loath to let me in. My quest began in February this year, after a report by the SPLC claimed that the number of Right-wing extremist groups had risen by 250 per cent since Obama’s election. I had approached groups from all over the country in the hope that one of them would let me watch them train. Then, in April, members of a radical Christian militia in Michigan known as the Hutaree were arrested for allegedly plotting to kill police officers. Suddenly, the movement had become even more fearful of media interest – not to mention the heat of the law.

I still wanted to find out first-hand why there were a growing number of people wanting to arm themselves – and against what exactly? And this was how, one spring evening, I eventually found myself face-to-face with Johnny Cochran, the head of Fireteam Diamondback (named after the rattlesnake abundant in this part of Texas), in his local Italian in the oil-rich city of Odessa.

I was due to watch Cochran and his men train the next day. But first, I wanted to find out more about him and his members. Why had he left the military? Cochran told me that he had been shot in the leg by friendly fire in Iraq and in the finger by an Iraqi, and had left the navy ‘just before Comrade Clinton took office in 1992’.

So how could he ever envisage taking up arms against the US military? ‘If they take our guns away,’ he told me. ‘They already did it after Hurricane Katrina. They declared a national emergency then went through neighbourhoods disarming civilians. The National Guard units went house-to-house, physically body slamming an elderly woman to the ground, taking a .38 revolver from her even though she was telling them she needed it to protect her family.’

That was a Republican administration, I pointed out. ‘The Democrats are running towards socialism at 100 miles an hour and Republicans are only running 60,’ he said. ‘They’ll all get to the same damn place eventually. Our job as militia is to re-establish the government in a way George [Washington] and the boys intended. And to do that we can’t go and hide in the bushes; we have to take active participation in the overthrow that Thomas Jefferson point-blank told us was our duty as Americans.’

Cochran certainly sounded ready for some ‘active participation’, reeling off a list of items he always carries in his car ‘in case of emergencies’: an AR15 assault rifle, a minimum 300 rounds of ammunition, a knife, first aid kit, food for three days, combat boots, a Cold Steel curved knife (‘I can remove a human limb with that and the head of a white tail buck with one swat’), a Kimber 45 pistol and six spare magazines, a shotgun and military-issue MREs (meals ready to eat).

I asked when he thought this revolution might happen. ‘We’re anticipating something happening prior to the November elections because the Democrats know they’re on the way out.’

This may sound like some crackpot fantasy, but it’s one that’s undeniably gaining currency. According to the SPLC report, there were 147 ‘patriot groups’ in 2006; by 2009 there were 512. ‘There has been a stunning expansion in these groups,’ Mark Potok, a spokesman for the SPLC, tells me. ‘In addition, there was an 80 per cent rise in hardline anti-immigration groups and hate groups like the Klan and neo-Nazis.’

Cochran, who runs a small oilfield company, told me emphatically that his group was not racist and that, like a lot of militias, he resents being grouped together with race-hate groups by organisations such as the SPLC.

‘They say we’re Nazis but it’s ironic because we’re faith-based and the Nazis deplored religion,’ he said. ‘We’re pro-rights, and the Nazis removed as many rights as possible. All this has done is strengthen the core support.’

Certainly, the militias I spoke to all seemed to share the same preoccupations. One man recruiting for a new militia in Oklahoma told me he wanted to be ‘prepared to put down a tyrannical government’. A member of a group in Mississippi said that if the government ‘did something crazy’, like take away their guns, he couldn’t predict what people would do: ‘This could get real ugly, real quick.’

Their opposition to federal government is what distinguishes them from the militias of old, which were designed to aid the government, rather than fight them, in the event of a national emergency. Indeed, according to civil rights organisation the Anti-Defamation League (ADL), the militia movement claims to be the ‘militia’ enshrined in the Constitution but is not. The ADL says that they are simply private, unregulated paramilitary groups but that there is no federal law against their existence.

Cochran, who grew up within 20 minutes of the city of Midland, George W Bush’s hometown, was introduced to the movement by his father ‘as soon as I was big enough to carry a rifle’, at the age of 10. He joined the Junior Reserve Officer Training Corps in high school, after which he signed up to the military. Cochran’s wife (he won’t reveal her name) was born in ‘liberal Kansas’ – apparently a running joke in their family – and has degrees in Spanish, mathematics and chemistry. She is also an expert marksman.

Cochran told me that some of the close-quarters training would be ‘off-limits’ the next day – although he would evidently relent when it came to demonstrating knifing someone in the throat. ‘We do simulation but we don’t condone anything that is illegal,’ he tells me. ‘If someone is to show up at one of our exercises with an illegal weapon they’ll be turned away.’

The next morning I am waiting with Cochran at a restaurant five miles out of town. Inside the next door petrol station, you can buy cowboy hats, model buffalo heads, and John Wayne mugs and alarm clocks. A truck pulls up and two men get out wearing camouflage gear.

One is Steven Page, who will later help Cochran with his ‘knife’ demonstration. The other is ‘Shepherd’, a 48-year-old computer store owner. Both are from another group, the Southwest Texas Desert Militia, but occasionally train with Cochran. The sticker on the back of Shepherd’s truck reads: ‘I love my AR15’.

Cochran leads us to a ranch 10 miles up the road, owned by a friend in the oil business. The land here is flat and peppered with nodding donkeys. Black rubber piping snakes its way along the paths, carrying water to the wells: once the oil is extracted they are filled with water to prevent sink holes occurring.

We pull up by the edge of a large quarry. Grey clouds loom overhead and although it was 91F (33C) here the day before, today it’s a chilly 52F (11C). Cochran grabs his AR15 and starts walking in a straight line through the quarry, intermittently raising the gun to his eye and firing.

The ground is littered with hundreds of corroded steel and brass bullet cartridges. ‘Any damn fool can stand still to shoot a gun,’ he says. ‘You gotta be moving – and always keep your gun loaded; an empty rifle is a baseball bat.’ Shepherd walks forwards. ‘Safety off, mine is hot,’ he says, before taking eight shots at an old piece of wooden board 500 yards away on a bank. He walks another five paces and shoots again, this time blasting a large rock apart. ‘Rock o’clock,’ Cochran cackles.

Shepherd likes to quote the following to justify his involvement in the militia: ‘When seconds count, the police are always minutes away.’

‘Look at the Los Angeles riots – people were dying, man,’ Page adds.

A lot of their fears – and those of many militias like them – are of a kind of post-apocalyptic future in which the infrastructure of civilisation collapses. They say this could come about as a result of natural disaster or if the government imposes what they see as unconstitutional laws: enforced health care, increased gun control. And they want to be ready.

Cochran has even got a ‘safe zone’ – a ranch outside Odessa that has its own water source and solar panels to generate electricity, and where he stores food, guns and ammunition. ‘Jefferson himself anticipated a violent revolution every 75 to 100 years. We’re running a little behind schedule,’ he says. ‘The last thing I want to do is look down the sights of my rifle at another American – that would be the most sickening prospect I could dream of – however, I’m a realist. I can pray all day long that this won’t take place but I’m not stupid enough to think it never will.’

‘They’ll fine me for refusing to buy health care first,’ Shepherd says. ‘Then I won’t pay the fine, I won’t show up to court, they’ll come to try to take me to jail and at that point they’re on my land illegally. So someone’s going to be met at the door looking down the barrel. And it’ll only take one person to refuse to be taken away to start the whole thing off. I’m ready to put this into practice, I’m ready to lay down my life for what I believe.’ I don’t doubt him.

At the same time, Shepherd appears quiet and fairly gentle – not the sort of person you’d associate with a militia. I ask what his wife thinks of his involvement. ‘All she knows is what the mainstream media say about the militia – bad, bad, bad,’ he says. ‘She’s worried I’m going to get arrested but there are key things I never want to be a part of or that I will never talk about.’

Cochran tells me the militia has what are termed ‘standing orders’ in place, in the event they capture anyone attempting to impose martial law or take away their constitutional rights on the ‘battlefield’. American officers will be executed by bullet. Foreign fighters, mercenaries or civilians employed by the US government to carry out its work will be ‘hoisted and hung’.

Cochran walks to the back of his Hummer and begins to change into his ghillie suit – military clothing designed to blend in with the surrounding vegetation that is covered in strands of green and tan cotton fibres that hang off like matted fur. ‘Man, they are great colours, we wouldn’t see you 20 yards away in the mesquite,’ Shepherd says. Page adds: ‘Let us know if we pee on you.’

By the end of the day, Cochran has demonstrated how to stab someone wearing combat body armour, how to break the finger of someone pointing a gun at your head, and explained how to use a piece of PVC and some hand grenades (‘the most wonderful toys in the world’) to make a trip wire.

I ask Cochran who taught him all this. ‘Uncle Sam,’ he says, fixing me with a stare and then erupting in a throaty cackle before lighting another Marlboro. ‘And he spent a lot of money teaching me how to do this s---.’

We drive in convoy to a nearby café, where Shepherd prays over our chilli hot dogs. Cochran had told me his militia was a ‘faith-based organisation’ and after a chorus of ‘Amens’ I ask how they can reconcile practising killing people with their Christian beliefs. Wasn’t Jesus supposed to have been a pacifist?

Cochran is quick on the draw. ‘Jesus Christ said: “He who does not have a sword should sell his robe and buy one”, because a man who will not defend his family and friends is worse than a fool. Now, when Christ said that, a fool was absolutely the worst thing you could call someone.

Jesus said if a man is to strike you on the cheek, turn to him the other cheek. But if he strikes you on the other cheek, God leaves that up to you. You can either turn and walk away or you can fight.’ But he didn’t say that, I say. ‘No. But the catch is, after he strikes you on the other cheek, God doesn’t tell you what to do. It depends on how you’re struck,’ Cochran states.

The following day I call Mark Potok at the SPLC in Washington DC. He says the growth of these radical Right groups is related to three factors: the changing racial demographic of the country (by 2050 it’s estimated the US will lose its white majority); the election of Obama – who many of these groups feel does not represent the country their white Christian forefathers built; and a depressed economy.

Potok admits that not every group is racially motivated but says there is a ‘great deal of profound unease out there’. ‘It’s based on completely baseless fears of a new round of gun control, yet Obama has made it clear he’s not going to do that. He even signed a bill to allow guns to be carried in national parks,’ he says.

‘They say Obama has run roughshod over the Constitution by passing a health care bill, but so often these people argue something is unconstitutional when what they really mean is they don’t like it. If you don’t like it, vote out your congressman.’

So is Johnny Cochran’s outfit actually dangerous? ‘Some small percentage of members of these groups will act on their fears,’ Potok says. ‘I think when you get to the point of teaching people how to sever other human beings’ necks and carotid arteries, the law does get interested.’

He also says it’s entirely possible the US could see another ‘Oklahoma’, but when, and how, is impossible to predict. For the moment, that same ‘spark’ hasn’t happened. But it could – any day. And America needs to be ready.